Healthy middle-aged people who get enough sleep each night are less likely to accumulate calcium deposits in their coronary arteries, a sign of heart disease, than their more sleep-deprived peers.

In fact, an extra hour of sleep each night was associated with a 33% lower chance of coronary artery calcification, a reduction in heart risk that’s on par with having about a 16-point drop in systolic blood pressure, according to a study published in Wednesday’s issue of the Journal of the American Medical Association.

“There really is mounting evidence that there likely are subtle health consequences of really short nighttime sleep,” says Diane S. Lauderdale, PhD, of the University of Chicago, one of the authors of the study.

The study is the first to show this relationship and it can’t prove that short sleep actually causes artery clogging, so it must be confirmed by other research, Lauderdale cautions. And it’s not clear if trying to get more sleep can reduce coronary artery calcification. Nevertheless, she adds, “it’s probably a good idea to sleep at least six hours a night.”

In the study, called Coronary Artery Risk Development in Young Adults (CARDIA), the researchers looked at 495 men and women who were 35 to 47 years old and free of coronary artery calcification in 2000 or 2001. Five years later, 12.3% had signs of calcium accumulation in their heart arteries.

The volunteers wore a wristwatch-like device to track their movement overnight, a technique called actigraphy that’s a much more accurate way to measure sleep time than personal estimates. CT scans were used to measure calcium deposits in the coronary arteries. The volunteers got an average of five to seven hours of sleep a night.

The calcification risk declined steadily as the number of sleep hours increased, even after the researchers accounted for participants’ age, sex, race, level of education, whether or not they smoked, and whether or not they had sleep apnea, a sleep disorder that interrupts breathing and raises blood pressure and heart risk.

It’s not clear why less sleep might be harmful, or if factors that affect heart health might also affect sleep, Lauderdale says. “We really don’t have a very good understanding of what factors determine how much people sleep,” she adds.

It is known that blood pressure dips at night in healthy people, and that a lack of sleep can interfere with this natural process and boost a person’s 24-hour average blood pressure. People who sleep less may also have higher levels of the stress hormone cortisol, which is a risk factor for heart disease.

High blood pressure could be a factor, as could inflammation, notes Robert Detrano, MD, PhD, a radiologist at the University of California at Irvine who runs China California Heart Watch, a non-profit organization in Kunming, China. Dr. Detrano was not involved in the current study, but did read CT scans for the CARDIA study. “It’s like a lot of things in science; you have a report like this, you have a discovery like this, you get more questions than answers out of it,” he says.

James E. Gangwisch, PhD, an assistant professor at Columbia University Medical Center in New York City, called it a “very powerful study.”

In his own 2007 research, Gangwisch found that short sleep duration is associated with an increased risk of type 2 diabetes. People who slept five or fewer hours a night or nine or more hours a night were more likely to be diagnosed with diabetes than those who said they slept seven or eight hours a night.

One of the strengths of the new study is that it directly measured sleep, rather than relying on a person’s estimate of their sleep time. “They were able to get much more precise measurements,” he explains. “When you get those more accurate measures you have a lot more statistical power. You can have small sample sizes and still see the relationships there.”